Solid hollow core fuel for fusion-fission engine

ABSTRACT

A fuel pebble for use in a fusion-fission engine includes a buffer material and a fertile or fissile fuel shell surrounding the buffer material. The fuel pebble also includes a containment shell surrounding the fertile or fissile fuel shell. The containment shell includes silicon carbide. The fuel pebble further includes a composite material layer surrounding the containment shell and a cladding layer surrounding the composite material layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims benefit under 35 U.S.C. §119(e) of U.S.Provisional Patent Application No. 60/997,780, filed on Oct. 4, 2007,entitled “Hybrid Fusion-Fission Reactor,” and U.S. Provisional PatentApplication No. 61/130,200, filed on May 29, 2008, entitled “HybridFusion-Fission Reactor Using Laser Inertial Confinement Fusion,” thedisclosures of which are hereby incorporated by reference in theirentirety for all purposes.

STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSOREDRESEARCH OR DEVELOPMENT

The United States Government has rights in this invention pursuant toContract No. DE-AC52-07NA27344 between the United States Department ofEnergy and Lawrence Livermore National Security, LLC.

BACKGROUND OF THE INVENTION

Projections by the Energy Information Agency and currentIntergovernmental Panel on Climate Change (IPCC) scenarios expectworldwide electric power demand to double from its current level ofabout 2 terawatts electrical power (TWe) to 4TWe by 2030, and couldreach 8-10 TWe by 2100. They also expect that for the next 30 to 50years, the bulk of the demand of electricity production will be providedby fossil fuels, typically coal and natural gas. Coal supplies 41% ofthe world's electric energy today, and is expected to supply 45% by2030. In addition, the most recent report from the IPCC has placed thelikelihood that man-made sources of CO₂ emissions into the atmosphereare having a significant effect on the climate of planet earth at 90%.“Business as usual” baseline scenarios show that CO₂ emissions could bealmost two and a half times the current level by 2050. More than everbefore, new technologies and alternative sources of energy are essentialto meet the increasing energy demand in both the developed and thedeveloping worlds, while attempting to stabilize and reduce theconcentration of CO₂ in the atmosphere and mitigate the concomitantclimate change.

Nuclear energy, a non-carbon emitting energy source, has been a keycomponent of the world's energy production since the 1950's, andcurrently accounts for about 16% of the world's electricity production,a fraction that could—in principle—be increased. Several factors,however, make its long-term sustainability difficult. These concernsinclude the risk of proliferation of nuclear materials and technologiesresulting from the nuclear fuel cycle; the generation of long-livedradioactive nuclear waste requiring burial in deep geologicalrepositories; the current reliance on the once through, open nuclearfuel cycle; and the availability of low cost, low carbon footprinturanium ore. In the United States alone, nuclear reactors have alreadygenerated more than 55,000 metric tons (MT) of spent nuclear fuel (SNF).In the near future, we will have enough spent nuclear fuel to fill theYucca Mountain geological waste repository to its legislated limit of70,000 MT.

Fusion is an attractive energy option for future power generation, withtwo main approaches to fusion power plants now being developed. In afirst approach, Inertial Confinement Fusion (ICF) uses lasers, heavy ionbeams, or pulsed power to rapidly compress capsules containing a mixtureof deuterium (D) and tritium (T). As the capsule radius decreases andthe DT gas density and temperature increase, DT fusion reactions areinitiated in a small spot in the center of the compressed capsule. TheseDT fusion reactions generate both alpha particles and 14.1 MeV neutrons.A fusion burn front propagates from the spot, generating significantenergy gain. A second approach, Magnetic fusion energy (MFE) usespowerful magnetic fields to confine a DT plasma and to generate theconditions required to sustain a burning plasma and generate energygain.

Important technology for ICF is being developed primarily at theNational Ignition Facility (NIF) at Lawrence Livermore NationalLaboratory (LLNL), in Livermore, Calif. There, a laser-based inertialconfinement fusion project designed to achieve thermonuclear fusionignition and burn utilizes laser energies of 1 to 1.3 MJ. Fusion yieldsof the order of 10 to 20 MJ are expected. Fusion yields in excess of 200MJ are expected to be required in central hot spot fusion geometry iffusion technology, by itself, were to be used for cost effective powergeneration. Thus, significant technical challenges remain to achieve aneconomy powered by pure inertial confinement fusion energy.

SUMMARY OF THE INVENTION

According to the present invention, techniques related to fuel for afusion-fission nuclear engine which we term Laser Inertial-confinementFusion-fission Energy (LIFE), are provided. Such an engine is describedin more detail in our commonly assigned copending U.S. patentapplication Ser. No. ______, entitled “Control of a Laser InertialConfinement Fusion-Fission Power Plant,” filed contemporaneously withthis application, the contents of which are incorporated by reference.More particularly, an embodiment of the present invention provides anenhanced fuel pebble suitable for use in a laser inertial confinementfusion-fission power plant. Merely by way of example, the invention hasbeen applied to the design and fabrication of a robust solid hollow corefuel pebble capable of high burn-up. The methods and systems describedherein are also applicable to other nuclear power plant designs.

According to an embodiment of the present invention, a fuel pebble foruse in a fusion-fission engine is provided. The fuel pebble includes abuffer material (e.g, a porous carbon aerogel) and a fertile or fissilefuel that forms a hollow shell surrounding the buffer material. Thefissile fuel shell includes uranium oxy-carbide in a specificembodiment. The fuel pebble also includes a containment shellsurrounding the fertile or fissile fuel shell. The containment shellincludes silicon carbide (SiC). The fuel pebble further includes acomposite material layer surrounding the containment shell and acladding layer surrounding the composite material layer. The compositematerial layer includes a high-strength carbon fiber wrap, where thecarbon fibers are coated with protective material such as siliconcarbide (SiC), in a particular embodiment.

According to another embodiment of the present invention, a method offabricating a fuel pebble for a fusion-fission engine is provided. Themethod includes forming a fertile or fissile shell and enclosing abuffer material inside the fertile or fissile shell. The method alsoincludes forming a containment shell surrounding the fertile or fissileshell and wrapping the containment shell with a composite materiallayer. The method further includes forming a wear andcorrosion-resistant cladding layer surrounding the composite materiallayer. The cladding also helps forms the hermetic pressure boundary forfission gas containment.

According to an alternative embodiment of the present invention, a fuelpebble for use in a fusion-fission nuclear engine is provided. The fuelpebble includes a foam core and a fertile or fissile shell surroundingthe foam core. The fuel pebble also includes a containment vesselsurrounding the fertile or fissile shell and a composite material layersurrounding the containment vessel. The composite material layerincludes carbon fiber filaments with a protective coating. The yieldstress of the composite material layer is greater than the intrinsicstrength of silicon carbide (SiC), which has been measured to beapproximately 450 MPa for a particular type. The fuel pebble furtherincludes a cladding layer surrounding the composite material layer.

Numerous benefits are achieved by way of the present invention overconventional techniques. For example, the present technique provides arobust fuel for nuclear reactors that can achieve high burn-up offertile or fissile material without failure of the fuel pebble.Additionally, embodiments of the present invention provide a fuel pebblethat has a high mass fraction of fertile material. Moreover, wallstresses in fuel pebbles described herein are reduced at comparablefinal inventory of metal atoms burn-up levels in comparison withconventional fuels, such as tri-structural iso-tropic (TRISO) fuels.Depending upon the embodiment, one or more of these benefits may beachieved. These and other benefits will be described in more detailthroughout the present specification and more particularly below.

These and other objects and features of the present invention and themanner of obtaining them will become apparent to those skilled in theart, and the invention itself will be best understood by reference tothe following detailed description read in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified schematic diagram of a solid hollow core fuelpebble according to an embodiment of the present invention;

FIG. 2 is a simplified graph of stress as a function of radial distancefor a solid hollow core fuel pebble according to an embodiment of thepresent invention;

FIG. 3 is a simplified flowchart illustrating a method of fabricating asolid hollow core fuel pebble according to an embodiment of the presentinvention; and

FIG. 4 is a simplified flowchart illustrating a process for generationof fertile or fissile fuel according to an embodiment of the presentinvention.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

According to the present invention, techniques related to fuel for anuclear engine are provided. More particularly, an embodiment of thepresent invention provides an enhanced fuel pebble suitable for use in alaser inertial confinement fusion-fission power plant. Merely by way ofexample, the invention has been applied to the design and fabrication ofa robust solid hollow core (SHC) fuel pebble capable of high burn-up.The methods and systems described herein are also applicable to othernuclear power plant designs. Additional discussion related to nuclearfusion-fission engines is provided in U.S. patent application Ser. No.______, entitled “Control of a Laser Inertial Confinement Fusion-FissionPower Plant” (Attorney Docket No. 027512-000400) and U.S. patentapplication Ser. No. ______ (Attorney Docket No. 027512-000600US),entitled “TRISO Fuel for High Burn-Up Nuclear Engine”, the disclosuresof which are hereby incorporated by reference in their entirety for allpurposes.

The inventors have determined that the mass fraction of fertile materialin conventional compacts that include an inert material is limited bythe packing efficiency of the fuel particles that make up conventionalcompacts. As an example, in some conventional TRISO particles, thenon-fertile or fissile materials utilized to provide mechanicalstructure for the particle consume space that is not then available forfertile material. As a result, when the TRISO particles are compactedwith inert materials to form a fuel pebble, the mass fraction of fertilematerial in the pebble is limited.

Additionally, the inventors have determined that in some fuel particledesigns, the heat transfer from the fertile or fissile material to thecoolants is limited by the inert materials disposed between the fertileor fissile material and the coolant. As an example, for coolants locatedat the exterior surfaces of a fuel pebble, heat from the fissionreactions occurring in the fertile or fissile material must transitmaterials between the kernel and the coolant. For this example, the heattransfer properties are a function of particle and pebble design,limiting desirable heat transfer.

FIG. 1 is a simplified schematic diagram of a solid hollow core fuelpebble according to an embodiment of the present invention. In theembodiment illustrated in FIG. 1, the fuel pebble 100 is referred to asa solid hollow core (SHC) pebble because the core of the pebble ispreferably a porous or nano-porous foam material. The foam in the hollowcore of the pebble can be formed from carbon, metals or ceramics.Referring to FIG. 1, a buffer material 110 (e.g., a foam core) ispresent in the center of the fuel pebble. In a particular embodiment, asdescribed more fully below, the buffer material includes a metal foamthat is injection molded into the core of the fertile or fissile fuelshell. In some embodiments, this foam material provides a source ofsacrificial silicon carbide (SiC) as well as providing regions forstorage of fission gases generated in the fertile or fissile kernel 120via chemisorption on the surface of the foam. In the embodimentillustrated in FIG. 1, the kernel 120 is a fertile uranium oxy-carbide(UOC) shell that surrounds the foam material 110.

Because the foam core is disposed internally to the fertile shell 120and does not surround the fertile or fissile kernel, the foam materialdoes not result in insulation of the kernel from the coolants on theexterior of the fuel pebble. Thus, heat from the kernel produced duringfission processes is transported toward the coolant without passingthrough the foam core. Therefore, embodiments of the present inventionprovide improved thermal conductivity in comparison with conventionaldesigns because heat from the fission processes transfers outward fromthe kernel and the amount of insulating material through which the heatmust pass is reduced.

In an embodiment, the foam material is fabricated from a porous carboncore that provides an expansion volume for fission gases. In addition tothis porous carbon or other foam material, the core can include asacrificial silicon carbide (SiC) material. For example, the sacrificialsilicon carbide (SiC) can react with palladium produced as a fissionbyproduct to form Pd₅Si. The consumption of the palladium in the coreprevents the palladium from reacting with and thereby degrading thesilicon carbide (SiC) containment vessel, described more fully below.Additionally, the use of sacrificial silicon carbide (SiC) in the coreshould mitigate attack of the fuel materials by fission products. One ormore pyrolytic carbon layers can be included as part of the core 110 andmay serve as transition layers between the buffer material and thefertile or fissile shell, described more fully below.

The solid hollow core fuel pebble also includes a fertile or fertile orfissile shell 120 surrounding the foam core. The fertile or fissileshell can include a variety of materials such as metallic uranium,uranium dioxide (UO₂), uranium carbide, uranium oxy-carbide (UOC),uranium nitride, and various other forms of uranium; metallic plutonium,plutonium dioxide (PuO₂), plutonium carbide, plutonium oxy-carbide(PuOC), plutonium nitride, and other various forms of plutonium; andvarious forms of thorium, or the like. The fuel materials can originatefrom weapons grade plutonium, highly enriched uranium, light waterreactor spent nuclear fuels, depleted uranium, natural uranium, naturalthorium ore, or the like.

At the interface of the foam core and the fertile shell, it is possibleto form 1:3:3:5 U:Pd:Si:C as a result of the reaction of palladium anduranium from the fertile or fissile shell and silicon carbide (SiC) inthe foam core. 1:3:3:5 U:Pd:Si:C, which has a melting point of ˜1952°C., can serve to remove fission products, namely palladium, and is verystable. Other high-melting refractory compounds may also form during thereaction of this, or any other sacrificial material.

Surrounding the fertile shell is a multi-purpose layer or series oflayers 130 containing one or more of the following materials: siliconcarbide (SiC), zirconium carbide, and/or pyrolytic carbon. The siliconcarbide (SiC) will provide sacrificial material that will react withfission products such as palladium. The use of sacrificial materialsincluding silicon carbide (SiC) is discussed above. The zirconiumcarbide layer provides a diffusion barrier that reduces or preventsdirect contact of fission products from the fertile or fissile kernelwith the silicon carbide (SiC) containment shell 140. Additionally, ZrCcan serve as an oxygen getter to reduce the oxygen pressure due to thegeneration of free oxygen from, for example, UOC. The pyrolytic carbonlayer positioned between the hollow kernel and the silicon carbide (SiC)containment shell, can serve as a transition layer from the kernel tothe SiC shell, as described more fully below. The transition layerprovides interfacial benefits that prolong the fuel pebble's utility andlifetime.

The silicon carbide (SiC) sacrificial material, the zirconium carbidediffusion barrier, and/or the pyrolytic carbon transition layer areformed, either as sequential single layers or as a multi-layer stack inwhich each of the layers, which may be referred to as sub-layers, isdeposited one or more times in a periodic or non-periodic manner. Thus,for example, several layers of the zirconium carbide diffusion barriermay be deposited in conjunction with the other sub-layers to form themultipurpose “layer” 130.

The silicon carbide (SiC) containment vessel 140 is an annular shellsurrounding the interior layers and makes up a portion of a pressurevessel or structure that serves to contain the fission gases within thefuel pebble. In the design of the solid hollow core fuel pebbledescribed herein, the silicon carbide (SiC) containment vessel 140 doesnot have to bear all the pressure resulting from the fission gasesbecause the additional layers surrounding the silicon carbide (SiC)shell also contribute to the containment of the fission gases. Thus,embodiments of the present invention differ from some conventionaldesigns utilizing only a single silicon carbide (SiC) layer to containthe fission gas pressure. As described below, the series of layersillustrated in FIG. 1 provide either increased strength for the fuelpebble or a similar strength while utilizing less materials. Byutilizing a reduced amount of inert materials, the mass fraction offertile or fissile material is thereby increased as desired.

A layer of high-strength carbon fiber windings 150, where the fibers arecoated with a protective material such as silicon carbide (SiC), isillustrated in FIG. 1. Other protective coating materials are possible,including but not limited to zirconium carbide (ZrC). In someembodiments, layer 150 is referred to as a carbon fiber wrap. As will beevident to one of skill in the art, carbon fiber materials, alsoreferred to as composite materials, are characterized by failurestrengths measured in the GPa range. Thus, in some embodiments, the useof a SiC/SiC fiber wrap provides an increase in strength of up to orover a factor of five in comparison with the strength of the SiCcontainment vessel.

As illustrated in FIG. 1, in order to increase the strength of thefission gas containment vessel structure, the silicon carbide (SiC)containment shell 140 is wrapped using carbon fiber. The carbon fiber,typically in a string-like or filament form, is wound around shell 140in a particular embodiment to form a carbon fiber wound sphericalpressure vessel. Epoxies, resins, or other appropriate materials areutilized in forming the carbon fiber windings. As will be evident to oneof skill in the art, techniques applicable to the fabrication of carbonfilament wound pressure vessels such as self-contained breathingapparatuses used by firefighters and other emergency personnel, SCUBAtanks for divers, oxygen cylinders for medical and aircraft uses, fuelstorage for alternative fuel vehicles, and the like, will also beapplicable to embodiments of the present invention. One of ordinaryskill in the art would recognize many variations, modifications, andalternatives.

The size the partially fabricated fuel pebble at the fiber wrap shell issuitable for the application of a number of carbon fiber compositewrapping techniques. Thus, the relatively large size (˜2 to 4 cm) of thefuel pebble allows for the use of a silicon carbide (SiC) wrap thatresults in a very strong pressure vessel. Similar techniques may not beapplicable or suitable for smaller dimension fuels. In one wrappingtechnique, the spherical pebble is continuously and rotated on aturn-table, while the silicon-carbide coated carbon has binder appliedto its surface, and is continuously fed to the rotating pebble.

The high strength composite wrapped shell 150 is surrounded by atransition layer 160 that provides a smooth surface in comparison withthe fiber wrap layer, which will generally have variations due to thefilament size used during the wrapping process. A variety of materialscan be utilized in forming the fiber-to-clad transition layer 160,including: graphite silicon carbide (SiC), zirconium, zirconium carbide(ZrC), refractory metals, refractory metal carbides, ferritic steels, orthe like. These materials can be applied as single layers, or asmulti-layer structures.

The fuel pebble includes a corrosion resistant cladding layer 170 madeof a material that is compatible with molten salt coolants, whichinclude but are not limited to refractory metals such as tungsten andvanadium. Molten salt coolants, including FLIBE (2LiF+BeF₂=Li₂BeF₄ andFLINABE (LiNaBeF₄), can attack the fuel pebble, reducing useful lifetimeof the pebble. Therefore, the cladding layer material is characterizedby a resistance to attack from the molten salt coolant(s) utilized toremove heat from the fuel pebble. Additionally, tritium fluoride, whichbehaves like hydrofluoric acid, is formed in the molten salt coolant asa result neutron bombardment and the consequent transmutation of lithiumthat comprises the salt. Therefore, the cladding layer 170 is alsoselected to be resistant to attack by hydrofluoric acid. Accordingly,embodiments of the present invention utilize cladding layers includingrefractory metals such as tungsten and vanadium, refractory-metalcarbides, or the like.

FIG. 2 is a simplified graph of stress as a function of radial distancefor a solid hollow core (SHC) fuel pebble according to an embodiment ofthe present invention. As illustrated in FIG. 2, the tangential stressis plotted as a function of radial distance, starting in the center ofthe foam core 110, passing through the hollow fertile or fissile kernel120, the multipurpose layer 130 (not marked in the figure for purposesof clarity, the silicon carbide (SiC) containment vessel 140, and theouter layers including the composite wrap and cladding layers. Thetangential stress is plotted as a function of the burn-up measured inpercent Fissions per Initial Metal Atom (FIMA). As shown in FIG. 2, thefoam core experiences a minimal amount of stress, either compressive ortensile, during the entire burn-up cycle. The hollow kernel 120experiences increasing compressive stress as the burn-up cycleprogresses, eventually reaching a level of greater than −300 MPa at thefoam-core/fuel-kernel interface at 99.9% FIMA. As the hollow kernelbegins to fracture at very high levels of burn-up, this compressivestress serves to hold the curved pieces of the fractured kerneltogether, thereby preventing the fuel pebble from failing.

The silicon carbide (SiC) containment vessel 140 is under increasingtensile stress as the burn-up cycle proceeds to full (e.g., 99.9%)burn-up. Although the wall stress increases to approximately 150 MPa asa result of radioactive fission gases such as krypton-85 generated inthe fertile or fissile kernel, the wall stress is well below the yieldstress of SiC, which in some cases is approximately 450 MPa. Asillustrated, the stresses in the composite wrap layer are even lowerthan in the SiC containment vessel layer, and are far below the yieldstresses of such composite layers, which can be in GPa range. Thus, thefuel pebble described herein provides for containment of fission gaseswithout pebble failure throughout the entire burn-up cycle, i.e., 99.9%FIMA.

FIG. 3 is a simplified flowchart illustrating a method of fabricating asolid hollow core fuel pebble according to an embodiment of the presentinvention. In the embodiment illustrated in FIG. 3, the fertile orfissile fuel is recycled from light water reactor (LWR) spent nuclearfuel (SNF) 310. The LWR SNF is processed using the DUPIC (Direct Use ofspent PWR fuel in CANDU reactors) process 312, in which the claddingfrom the fuel rods and the fission gases are removed. An OREOX(Oxidation, REduction of enriched OXide fuel) process 314 is then usedto produce a powder form of uranium dioxide (UO₂) 316. A carburizationprocess 318 is then used to generate a powdered form of uraniumoxy-carbide (UOC) 320. It should be noted that although UOC is utilizedas the fertile or fissile kernel in some embodiments, this is notrequired by the present invention. In other embodiments, other fertileor fissile materials are utilized depending on the particularapplication. Thus, although a process to generate UOC from LWR SNF isillustrated in steps 310-320, this particular fuel process is notrequired and other fuels can be generated and utilized in the solidhollow core fuel pebble described herein. Accordingly, other fertile orfissile materials including weapons-grade plutonium (WG-Pu), natural anddepleted uranium (DU), highly enriched uranium (HEU), and the like canbe utilized in forming the fertile or fissile kernel used in variousembodiments. One of ordinary skill in the art would recognize manyvariations, modifications, and alternatives.

FIG. 4 is a simplified flowchart illustrating a process for generationof fertile or fissile fuel according to an embodiment of the presentinvention. It is likely that the most logical solid fuel option wouldleverage as much of the DUPIC fuel cycle as possible, as illustrated inFIG. 4. As described in the literature, the key process of the DUPICfuel cycle is the oxidation and reduction of PWR spent oxide fuel (knownas OREOX) to prepare powder for CANDU fuel fabrication. This is acompletely dry process without any separation of fertile or fissileisotopes from the spent PWR fuel. It is assumed that the composition ofPWR spent oxide fuel fed to the OREOX process consists of U, Pu, Np, Am,Cm and miscellaneous fission products. The composition of the powderleaving the OREOX process for fabrication of the CANDU fuel consists ofU, Pu, Np, Am, Cm and miscellaneous fission products. This is also thecomposition of dry powder available from PWR SNF for fabrication of fuelas described herein.

The primary waste stream coming from the DUPIC fuel fabrication processconsists of metallic components from spent PWR fuel, fission gases, andsemi-volatile fission products released from the fuel during treatment.Noble gases such as Kr and Xe are compressed in 50-liter cylinders forlong-term storage and decay. Tritium and carbon are trapped on molecularsieves and barium hydroxide, respectively. These are then mixed withcement, which is poured into large drums for disposal. The discardedcladding (hulls) are also mixed with -cement for disposal. Radioactiveiodine is trapped on a silver zeolite and Cs and Ru are fixed on filtersand vitrified for disposal. As described in relation to FIG. 3, theinventors have developed computational models for solid hollow core fuelthat account (to the extent possible) for the effects of fuelirradiation on materials stress and failure. The inventors havedetermined that for the solid hollow core fuel pebbles described herein,the stresses, due to fission gas accumulation, thermal gradients, andirradiation-induced swelling of the materials can be maintained belowlevels that would result in failure of the primary silicon-carbide (SiC)pressure vessel or boundary 140 illustrated in FIG. 1.

Referring once again to FIG. 3, a slurry is formed 330, typicallyutilizing a mixture of silicon carbide (SiC), polyethylene glycol (PEG),or other binder, and fertile or fissile powder. The slurry is placedinto a mold 332 and a hot isostatic pressing (HIP) 334 process isutilized to form two hemispherical shells 336 of fuel that have hollowcentral regions. Isotropic compression may be achieved hydrostatically,with the slurry contained within a flexible membrane. As will beappreciated with reference to FIG. 1, the fuel kernel shell will befabricated in subsequent processing steps by joining the twohemispherical sections together with the hollow core between thesections.

In order to form the fuel shell, the two hemispherical sections are fittogether 340 and an injection molding process is used to form the foammaterial at the center of the solid hollow core fuel pebble. As anexample, a resorcinol-formaldehyde (RF) or other solution can beinjected into the center of the hollow fertile or fissile fuel kernel,thereby enabling in situ formation of a carbon aerogel during heatingand consequent pyrolysis 344, Small holes in the hollow kernel enablethe release of pyrolysis gas from the hollow core. The carbon aerogelcould also be placed in the hollow core by other injection moldingprocesses. After filling the hollow core of the kernel with carbon foam,the resulting part is known as a foam-core/fuel-kernel combination.Although the embodiment described above forms the fuel shell and theninjects the buffer material into the fuel shell, this is not required byembodiments of the present invention. In other process flow, the buffermaterial is formed and the fuel shell is then positioned to surround thebuffer material. One of ordinary skill in the art would recognize manyvariations, modifications, and alternatives. For example, the carbonaerogel foam could be formed in the two kernel hemispheres beforeassembly into SHC kernel.

In order to form the multipurpose layer 130, the foam-core/fuel-kernelcombination is placed into a chemical vapor deposition (CVD) reactor 346in which one or more layers are deposited. The CVD reactor may utilize areduced or atmospheric pressure, plasma enhancement, or the like.

The silicon carbide (SiC) sacrificial material, the zirconium carbidediffusion barrier, and/or the pyrolytic carbon transition layer areformed, either as sequential single layers or as a multi-layer stack inwhich each of the layers, which may be referred to as sub-layers, isdeposited one or more times in a periodic or non-periodic manner. Thus,for example, several layers of the zirconium carbide diffusion barriermay be deposited in conjunction with the other sub-layers to form themultipurpose “layer.”

Either the same or a different CVD reactor is utilized 348 to form theprimary silicon carbide (SiC) containment shell, or pressure boundary140. In some embodiments, the interfaces between various layers of thestructure are not exposed to an ambient environment during the CVDprocess, improving the fuel performance. Thus, in some embodiments, asingle CVD reactor is utilized with multiple gas sources. In otherembodiments, multiple CVD reactors joined by a load-lock vacuuminterface can be utilized to achieve results similar to those achievedwith a single reactor. The high-strength carbon fiber composite wrap orwinding process 360 is then utilized to form the composite wrap layer150. In an embodiment, high-strength carbon fibers or filaments, coatedwith a protective material such as silicon carbide (SiC), are utilizedto form the wound pressure vessel. Other protective coating materialscan also be used on the surface of the fibers.

After the winding process, a carburization process 362 is utilized,followed by formation of the cladding 364, typically using refractorymetals. As illustrated in FIG. 3, a physical vapor deposition (PVD)process is utilized to deposit refractory metals such as tungsten orvanadium. Various PVD processes including electron-beam evaporation, DCmagnetron sputtering, or the like can be utilized depending on theparticular materials to be deposited. One of ordinary skill in the artwould recognize many variations, modifications, and alternatives. Forexample, the spherical cladding can be formed from two pre-existingfree-standing hemispherical shells.

It should be appreciated that the specific steps illustrated in FIG. 3provide a particular method of fabricating a solid hollow core fuelpebble according to an embodiment of the present invention. Othersequences of steps may also be performed according to alternativeembodiments. For example, alternative embodiments of the presentinvention may perform the steps outlined above in a different order.Moreover, the individual steps illustrated in FIG. 3 may includemultiple sub-steps that may be performed in various sequences asappropriate to the individual step. Furthermore, additional steps may beadded or removed depending on the particular applications. One ofordinary skill in the art would recognize many variations,modifications, and alternatives.

For example, one variation is that the fuel pebbles can be filled withpowders of fertile or fissile materials, including, but not limited to:uranium, thorium, plutonium; oxides of uranium, thorium, and plutonium;carbides of uranium, thorium, and plutonium; oxy-carbides of uranium,thorium, and plutonium; nitrides of uranium, thorium and plutonium; andother chemical forms of uranium, thorium and plutonium. In suchimplementations vibration can be used during filling to maximize thepacking density of particles within the hollow pressure boundary. Thisembodiment has the advantage of eliminating the need for fertile orfissile particle consolidation, and hot isostatic pressing. In addition,thermally-conductive carbon, metallic or ceramic powders can be added tofill the interstitial spaces between the fertile or fissile materialpowders, thereby enabling improved thermal conductivity and relativelyhigh rates of heat transfer. These thermally-conductive powders have aparticle-size distribution typically smaller than that of the fertile orfissile material powders, and can be selected to enable in situsintering of the fuels during a “fuel formation” or “fuel burn-inphase.” Vibration can be used during filling to maximize the packingdensity of particles within the hollow pressure boundary.

It is also understood that the examples and embodiments described hereinare for illustrative purposes only and that various modifications orchanges in light thereof will be suggested to persons skilled in the artand are to be included within the spirit and purview of this applicationand scope of the appended claims.

1. A fuel pebble comprising: a buffer material; a fissile fuel shellsurrounding the buffer material; a containment shell comprising siliconcarbide surrounding the fissile fuel shell; a composite material layersurrounding the containment shell; and a cladding layer surrounding thecomposite material layer.
 2. The fuel pebble of claim 1 wherein thebuffer material comprises a porous carbon material.
 3. The fuel pebbleof claim 2 wherein the buffer material further comprises at least one ofa silicon carbide material or a pyrolytic carbon material.
 4. The fuelpebble of claim 1 wherein the fissile fuel shell comprises a uraniumoxy-carbide material.
 5. The fuel pebble of claim 1 further comprising adiffusion barrier surrounding the fissile fuel shell.
 6. The fuel pebbleof claim 5 wherein the diffusion barrier comprises a zirconium carbidematerial.
 7. The fuel pebble of claim 6 wherein the diffusion barrierfurther comprises at least one of a silicon carbide material or apyrolytic carbon material.
 8. The fuel pebble of claim 1 wherein thecomposite material layer comprises a carbon fiber layer.
 9. The fuelpebble of claim 1 wherein the cladding layer comprises at least one of arefractory metal or a metal carbide material.
 10. A method offabricating a fuel pebble comprising: forming a fissile shell; enclosinga buffer material inside the fissile shell; forming a containment shellsurrounding the fissile shell; wrapping the containment shell with acomposite material layer; and forming a cladding layer surrounding thecomposite material layer.
 11. The method of claim 10 wherein enclosingthe buffer material inside the fissile shell comprises injection moldinga foam material.
 12. The method of claim 11 wherein the foam materialcomprises a carbon aerogel material.
 13. The method of claim 10 whereinthe buffer material further comprises a sacrificial silicon carbidematerial.
 14. The method of claim 10 further comprising forming at leastone of a sacrificial layer, a diffusion barrier, or a transition layersurrounding the fissile shell at a position interior to the containmentshell.
 15. The method of claim 10 wherein forming the containment shellcomprises depositing silicon carbide using a chemical vapor depositionprocess.
 16. The method of claim 10 wherein the composite material layercomprises carbon fibers coated with silicon carbide. 17-18. (canceled)19. The method of claim 16 wherein a yield stress of the compositematerial layer exceeds 450 MPa.
 20. The method of claim 10 whereinforming the cladding layer comprises depositing the cladding layer usinga physical vapor deposition process.
 21. The method of claim 18 whereinthe cladding layer comprises a refractory metal.
 22. (canceled)
 23. Afuel pebble comprising: a foam core; a fissile shell surrounding thefoam core; a containment vessel surrounding the fissile shell; acomposite material layer surrounding the containment vessel andcomprising carbon fiber filaments, wherein a yield stress of thecomposite material layer is greater than 450 MPa; and a cladding layersurrounding the composite material layer.
 24. The fuel pebble of claim23 further comprising a zirconium carbide diffusion barrier disposedbetween the fissile shell and the containment vessel.
 25. The fuelpebble of claim 23 wherein the foam core comprises an RF carbon aerogel.26. The fuel pebble of claim 23 wherein the fissile shell comprisesuranium oxy-carbide.
 27. (canceled)
 28. The fuel pebble of claim 23wherein the fuel pebble is characterized by a mass fraction of fissilematerial greater than 10%. 29-32. (canceled)